1. Technical Field
The present invention relates to a construction machine including an electric turning control unit that drives a turnable body, and a method of controlling a turning electric motor.
2. Description of Related Art
Construction machines such as shovels often include a turnable body (referred to as upper-part turning body) in order to perform work while turning work mechanisms such as an arm and a boom. A turning mechanism is provided in order to turn the turnable body. A turning electric motor may be used in place of a hydraulic motor as a power source of the turning mechanism.
Proportional integral control (PI control) is often used to control driving of the turning mechanism. According to control of the turning electric motor based on PI control, the torque of the turning electric motor is controlled by integrating deviations between a speed command concerning the turning speed of the turnable body and an actual turning speed. In this case, decelerating and stopping the turnable body turning at a certain turning speed may cause the phenomenon that the turnable body stops near the stop position while swinging back and forth. This swinging is referred to as swing-back. There are several causes of the swing-back, one of which is attributable to the above-described PI control.
At the instant when the turning speed becomes zero, the torque of the turning electric motor does not become zero and the turning electric motor is outputting a torque pertaining to integral control. Assuming that the turning plane of the turnable body is a horizontal plane, the torque pertaining to integral control is a torque whose direction is opposite to the turning direction. This torque causes the turnable body to turn in the opposite direction after becoming stationary (becoming zero in speed). Then, the direction of the torque pertaining to the integral control also is reversed, so that the turn is controlled and the speed becomes zero. Then, again, a torque is generated in an opposite direction. Thus, while swinging back and forth near the stop position, the turnable body attenuates its amplitude and stops. This swinging is a swing-back.
In order to control the above-described swing-back, it has been proposed to reset the torque pertaining to integral control to zero at the instant when the speed of the turnable body becomes zero.